1. Field of the Invention
The present invention concerns a method and a device for detection of the position of an examination person in a magnetic resonance system, in particular for detection of the position and/or the orientation of an examination person on a horizontal table in a magnetic resonance system.
2. Description of the Prior Art
In an imaging magnetic resonance measurement or data acquisition (MR measurement) the quality of the images, the density of the slices in which imaging MR measurements are made and the speed with which the MR measurement can be executed increase with the radiated radio-frequency power (RF power). Only a certain maximum RF power can be used due to the absorption of the radio-frequency radiation (RF radiation) in the body tissue. The maximum allowable absorption is legally established with limit values for the specific absorption rate (SAR) that differ according to various sub-regions of the human body.
During an MR measurement the radiated RF power is continuously monitored in order to not exceed the legally dictated SAR limit values. Depending on the body regions exposed in the magnetic resonance system (MR system), the SAR monitoring determines the limit value for the allowed RF power and continuously monitors compliance therewith.
During an MR measurement, only the position of the table on which an examination person lies can be deduced. Additional information about the position of the examination person on the table (initial and end positions of the body) and about the orientation of the examination person (head-first or feet-first) is therefore required for the determination of the exposed body regions. Only with such information is it possible for the SAR monitoring to exactly determine which body regions will lie in which table position within the apparatus for a planned MR measurement.
The problem is further aggravated in the case of MR measurements in which the table is moved during the MR measurement (what are known as move during scan measurements, MDS measurements). During these MR measurements the exposed body regions must be continuously determined using the current table positions in order to determine and to monitor a possibly different limit value for the RF power.
The problem thus exists that information about the position and orientation of the examination person on the table is required for the determination of the exposed body regions and the monitoring of the SAR limit values.
Conventional MR systems are presently not designed to automatically establish the position and orientation of an examination person.
The present problem is addressed in part in MR systems according to the prior art by the operator being required to manually input the orientation of the examination person in the registration of the examination person. Although the body size of the person can likewise be input, this field is optional and, based on experience, is seldom filled out. However, the exact position of the body ends of the examination person on the table is not known even given input of the body size. In particular the precise knowledge of the head position of the patient is of importance because the lowest SAR limit value (which most severely limits the allowed RF power) exists for this body region.
In these conventional MR examinations missing information is determined through plausibility assumptions. For example, given an orientation with the head first it is assumed that the head lies at the position of the head coil when this coil is connected. The probable body size of the examination person is determined from statistical population data using the examination person's age, which is likewise to be filled out as an obligatory field in the registration. Assumptions about the position of the various body regions relative to the MR system are made by the SAR monitor on the basis of the probable body size. These assumptions can be very imprecise since the actual size of the examination person can deviate severely from the statistical mean. Moreover, these assumptions are no longer reliably possible given a feet first orientation. A worst-case estimation of the SAR limit value for the entire body is simply implemented. Due to the necessary safety tolerances, this estimation generally leads to too-low allowable RF powers that are too low, and thus significant limitations in the capability (number of slices, flip angle) of the appertaining MR measurements are made.
In other MR systems in the prior art, the aforementioned problem is also circumvented by simplified SAR models are used to determine the maximum allowable RF power. These models are largely independent of the position and orientation of the examination person. Due to the lack of discrimination of the exposed body regions, only a lower RF power can generally be used than would be possible according to legal SAR limit values for the respective body region. The capability of the MR measurements in these conventional MR systems is thereby limited relative to MR measurements with the maximum legally allowed RF power.
European Patent Application EP 1382300 A1 discloses a method and a device for positioning a patient in a medical diagnosis or therapy apparatus. In the described method, an image of the examination person is acquired with an image acquisition apparatus and the positions of various body regions are automatically determined by image processing. A scan region for a MR measurement is subsequently automatically suggested. The system is very complicated and requires both additional apparatuses (such as image acquisition apparatuses and computers) and programs for image processing with automatic detection of body regions.
The orientation of the examination person is typically manually input and the position of the examination person is estimated on the basis of assumptions. The manual input requires time on the part of the user and interrupts her or his workflow.
Moreover, due to the imprecise knowledge of the exposed body region, lower SAR limit values are used than the legally dictated SAR limit values. The RF power that is thereby reduced entails significant disadvantages for an imaging MR measurement such as, for example, longer measurement durations, a lower number of slices that are measured in a specific time, or a reduced image quality.